What a fascinating question. I've not yet had to face this problem in a big way, but can make some suggestions from how I think that I developed adequate SA myself.
I suggest that this might be developed by setting a series of exercises, where possible ground based, designed each to develop individual aspects of SA. Then, in a similar manner to teaching basic flying, eventually aim to combine them into creating a bigger picture. So, here are some suggestions...
- Sit at an airfield with a scanner and photocopied local chart, sketching paths of aircraft based upon their radio calls.
- Whilst flying or driving monitor trees / clouds / smoke / crops... to try and maintain a continuous knowledge of wind direction.
- Watching other airceaft, keep trying to guess what they'll do next, and why?
I've not tried this, but it seems to be an emulation of how I learned SA.
G